Many viruses have developed strategies to either infect or traverse through the epithelial cells to establish infection in the host. Most of them are specific for a particular epithelium. For example, rotaviruses infect intestinal epithelial layers. Papillomaviruses chronically infect epidermal layers. About one-third of Human Papillomaviruses specifically infect the genital tract. Human respiratory syncytial virus (RSV) infects the superficial layer of the respiratory epithelium. Influenza viruses infect the pulmonary epithelial cells. While the Human Immunodeficiency Virus (HIV) infection is initiated primarily by affecting the epithelial cells in the oral, rectal or genital mucosa, the virus infects immune cells like macrophages or CD4+ T lymphocytes.
Significant strides in the development of HIV/AIDS vaccine candidates that are immunogenic in humans and nonhuman primates have been made. However, the goal of achieving protective immunity against HIV infection remains elusive. The nature of the HIV virus has created several barriers to effective immune control by the humoral and cellular forms of adaptive immunity. These barriers include the antigenic variability of the virus; its ability to generate antigen-escape variants; its inherent resistance to development of and targeting by neutralizing antibodies; down regulation of MHC class I and CD4 on infected cells; and, preferential destruction of viral-specific CD4+ T lymphocytes.